Thieving abilities explained

When a rogue attempts to use one of his thieving abilities, his chances of success depend on several factors: his skill, the difficulty of the task at hand, and sometimes also his luck. Each thieving task has an associated difficulty score. For example, very complex locks or traps have higher difficulty scores than simpler ones.

The method used to determine the result (success or failure) of a thieving task is different for each thieving ability.

FINDING TRAPS, DISARMING TRAPS AND OPENING LOCKS
When the detect traps/illusions mode of a thief is active and there are traps within his line of sight, the game follows these steps to determine whether he detects each trap:

Roll a d10 and adjust the result with the rogue’s luck. This means that the luck score is added to the die roll, but results higher than 10 are reduced to 10, and results lower than 1 are increased to 1. Examples:

The luck of the rogue is 2. His die roll is 5. The adjusted result is 5 + 2 = 7.

The luck of the rogue is 2. His die roll is 9. 9 + 2 = 11, which is higher than 10, so the result is reduced to 10.

The luck of the rogue is -2. His die roll is 1. 1 - 2 = -1, which is lower than 1, so the result is increased to 1.

Add the adjusted result of the roll to the Detect traps score of the rogue.

Compare (2) with the difficulty of each trap. If (2) equals or exceeds the difficulty, the rogue detects the trap. If it’s lower, the rogue misses it.

There is a delay from the moment the thief activates the detection mode until he actually performs his first detection attempt. The duration of this delay is a random value between 0 and 6.6 seconds. After that, the game keeps performing this check once per round while the detect traps/illusions mode is active and there are traps nearby.

Example: A rogue with a score of 50 in Detect traps and +2 in luck is close to two traps with difficulty scores of 55 and 60. He activates his detect traps mode. The game rolls 1d10 and the result is 4. The roll adjusted with luck is 4 + 2 = 6. The adjusted roll is added to the detect traps score, for a total of 56. 56 is higher than the difficulty of the first trap (55), but lower than the difficulty of the second one, so the rogue detects only the first trap. The rogue keeps detecting. The game rolls again, this time the result is 9. The roll adjusted with luck is 10 (9 + 2 = 11, which is reduced to 10). The adjusted roll is added to the detect traps score, for a total of 60. 60 equals the difficulty of the second trap, so the rogue detects it as well.

The same method is used to determine the result when a rogue attempts to disarm a trap or open a lock. Some locks are impossible to pick and some traps impossible to detect or disarm regardless of the skill of the thief.

DETECTING ILLUSIONS AND SETTING TRAPS
The Detect illusions and Set traps abilities work in a similar way to the previous ones, but there are two important differences: The difficulty of the task is always 100, and the die rolled is a d100, which means that there is much more randomness involved.

When a rogue attempts to detect illusions the game rolls 1d100 and adds the result to his Detect Traps/Illusions score. If the result is lower than 100, the detection fails. Otherwise, the detection succeeds, all the illusion spells
within visual range of the character are dispelled, and any illusory creatures in that range are destroyed. The game keeps performing this check once per round while the detect traps/illusions mode is active and there are illusions nearby.

There is a delay from the moment the thief activates the detection mode until he actually performs his first detection attempt. The duration of this delay is a random value between 0 and 6.6 seconds. After that, the game keeps performing this check once per round while the detect traps/illusions mode is active and there are traps nearby.

Set traps works in the same way, but the die roll is adjusted with luck (see Detecting traps, disarming traps and opening locks above). Critical failures can happen when setting traps. On a critical failure the thief gets damaged by his own trap, although the damage he receives is not the same as the damage dealt by a successfully set trap. The special snares of Bounty Hunters are not subject to critical failures. However, a score of 100 or more in Set snare means automatic success, with no chance of critical failure.

STEALTH
Just like with detecting illusions and setting traps, the difficulty of stealth is always 100. The calculation, however, is more complex. The game makes this calculation:

1d100 MOD luck is the result of rolling 1d100 and adjusting the roll with luck (see Detecting traps, disarming traps and opening locks above). If the result of this adjusted roll is 1, it is a critical failure. In this case, the stealth attempt fails regardless of the skill scores.

Environment Multiplier is a variable that depends on how well lit the place where the rogue hides is. It has three possible values: Shadows or otherwise unlighted area: 1. Indoor lighted areas: 0.67. Daylight: 0.5.

If the result of the calculation equals or exceeds 100, the stealth attempt succeeds, otherwise it fails.

Critical failures: If the result of the adjusted roll is 1, it is a critical failure. In this case, the stealth attempt fails regardless of the skill scores of the rogue. Rogues with positive luck (1 luck or higher) never get critical failures, since they cannot possibly get 1 in the adjusted roll. Rogues with neutral (0) luck have a 1% chance of critical failure. Each point of negative luck increases the chance of critical failure by 1%. For example, a rogue with -2 luck has a 3% chance of critical failure.

PICKING POCKETS
When a rogue attempts to steal an item from a creature or a store, three results are possible:

Failure: The target notices the stealing attempt and turns hostile, and the rogue doesn't manage to steal anything.

The outcome is determined in differents ways depending on whether the rogue is stealing from a store or a creature.

STEALING FROM A STORE
The game makes this calculation:

(Thief skill - target skill) / 5 vs. 1d20

I. e., it calculates the difference between the Pick pockets score of the thief and the Pick pockets score of his target, divides it by 5, and compares the result with a d20 roll. If the result is equal or greater than the d20 roll, the attempt succeeds. Otherwise it fails (and the target turns hostile).

STEALING FROM A CREATURE
The process is more complex when stealing from creatures. The game starts by doing the same calculation described for stores, but in this case a roll of 20 is a critical failure, i. e., the attempt fails regardless of the skill of the rogue. Just like with store stealing, if the d20 roll check fails, the attempt fails and the target turns hostile. However, if the d20 roll check succeeds, it doesn't necessarily mean that the rogue manages to steal something, it just means that his attempt goes unnoticed. The game still needs another step to determine whether he actually steals something.

Each of the inventory slots of a creature has an associated pickpocketing difficulty score. If the target has an item in an inventory slot with a difficulty score equal or lower than the Pick pockets score of the rogue, he steals that item. Otherwise you get a message informing you that the target has no items a thief of your ability can get (and nothing happens).

Example: A thief with a Pick pockets score of 50 attempts to steal from a noble (Pick pockets score 0) who is wearing a ring on his finger. The game rolls a d20 and the result is 7. The first calculation ((Thief skill - target skill) / 5) yields: (50-0)/5=10. This is greater than the d20 roll, so the attempt doesn't fail. However, the pickpocketing difficulty score of worn rings (i. e., rings located in one of the ring slots of the inventory), is 60. The pickpocketing skill of the thief is lower than this, so he doesn't steal the ring. Then the thief drinks a potion of Master thievery, which raises his score to 90, and tries again. The game rolls a d20 and the result is 7 again, so he still avoids failure. This time his score is higher than the difficulty score of worn rings, so he succeeds and steals the ring.

The pickpocketing difficulty score of each inventory slot is:

Helmet, armor, shield and boots: Cannot be stolen.

Selected weapons and ammunition: Cannot be stolen.

Not selected weapon: 95.

Gloves, amulet, belt and cloak: 80.

Rings: 60.

Not selected ammunition and quick items: 50.

Other inventory slots and gold: 10.

Hostile creatures cannot be pickpocketed. However, using a charm like spell to make them friendly makes them vulnerable to pickpocketing.

Some conclusions

A rogue needs a Pick pockets score at least 5 points higher than his target just to have any chance of success at all. If the difference is lower, his stealing attempts will automatically fail.

A rogue attempting to steal from a store has guaranteed success if his Pick pockets score is at least 100 points higher than the Pick pockets score of the seller.

There's no way to have guaranteed success when stealing from a creature.

This post was originally a question about thieving abilities. I leave the original question inside the spoiler for reference:

Each thieving ability has a score, but I don't know the meaning of those numbers. What does it mean if I have a score of 50 in opening locks? Or a score of 150 in picking pockets? What does the game do with those numbers?

Comments

My understanding is that you roll 1d10, add that value to your overall score (resulting from the points you've invested ± racial modifiers ± DEX modifiers ± item/potion modifiers), and the final number is checked against the "difficulty" of the task at hand. If your number matches or exceeds the difficulty, you are successful.

Not sure how the difficulty of moving silently/hiding in shadows is determined at any given point in time, though.

afaik for moving silently/hiding in shadows an average of the 2 values is used, and the old belief that moving silently was increasing the time the thief remain hidden and h.i.s. not is proven false, theu work the same way. then the value, + modifiers and dice roll is used against the light value of the specific point of the map where the thief is at the moment that try to hide. if the thief is successful he remain hidden, unless revealed by spells or other, for a certain time, also depending by the averaged value of the 2 skills, and can move in full light places with no problem.

edit: my approach is a trial and error one, i don't look into the game files with modding tools, so i can not tell anything on the dice roll used, that can be the same 1d10 or different. but as i hide intensively my thieves i am sure that the light greatly affect the chance of success and that the more i increase the skills the more time pass before the next check, that is done using the light value of the point where the thief is in that moment, not the one where he originally hided.

@Alonso i really think that who has knowledge sometimes can give it to us in a less cryptic form, using terms everybody can understand (like a doctor can tell us " you have a cold" instead of "you have a rhinitis", "rhinitis? it is a serious illness? will i die?" ). some doctors are terrible about that and use on purpose cryptic terms to create a distance between them and the one they have to cure, making him an object of their "magic" instead of the protagonist of the healing process, i had learned it in the college, is one of the first things they teach you when you study medicine (at least in my country)...
but i don't think that here is the case, i think that to have a deep and exhaustive insight on the kind of things you are asking about in this and other threads you have to learn at least some basics of the high elfic, ie to know what XXXX.2DA mean, and the base syntax used programming the IE games.
because a real understanding of the matter, that is so complex at this level of detail, can only be obtained knowing how the engine works, what is programmed to do.

@gorgonzola: I think I'm a bit unusual in the sense that I have a background both as a programmer and as a teacher. That means that I have spent years dealing with very complicated and weird stuff (the programming), but I have also spent years figuring out how to explain things in a simple way (the teaching).

When I was in university I always liked to use big words to sound important. I remember this time I was upset because I had an assignment that was quite confusing. When I explained the problem to my friend, who also studied engineering, I started by saying "You know how Fourier transforms work, right? OK, so the problem is..." I felt so important using those words that only he and I could understand. However, the next day I wanted to explain the same thing to my mother, who doesn't know anything about big engineering words. So I just said "mum, that exercise was so confusing, all the important information was in the wrong place!". Of course, my mother understood the problem perfectly, there was no real need to use all the big words.

We programmers love so much these big words that we use them all the time until most of us lose our ability to talk about certain topics in plain English (or Italian, or whatever your language is). In my case I never lost the capacity to speak in plain English (or Spanish) because everybody in my family is a teacher, so I've been always encouraged to explain things the easy way. But if my relatives were also engineers, I'm sure I would have lost the ability to speak in plain English a long time ago.

An example of how anything in these games can be explained in plain English is my post on luck.

i did not know about your background. and indeed i like how you translated from elfic to common language in that thread, the op post that is the translated version of what the experts of ie told in elfic is easily understood by anyone.

Back on topic, I'll try to use my very rudimentary notions of high elfic and some imagination to attempt a translation of this into common:

When a rogue attempts to steal an item, his chances of success depend on his pickpocketing skill score and the location of the item within the inventory of his target. Each inventory slot has an associated pickpocketing difficulty score. The attempt to steal succeeds if the skill of the thief is equal or greater than the difficulty score of the item he's trying to steal, otherwise it fails.

Example: A thief with a pickpocketing skill of 70 attempts to steal from a noble who is wearing a ring in his finger. The pickpocketing difficulty score of worn rings, i. e., rings located in one of the inventory ring slots, is 60. Since the pickpocketing skill of the thief is greater than the difficulty score of the item he's trying to steal, he succeeds and steals the ring.

The pickpocketing difficulty score of each inventory slot is:

Helmet, armor, shield and boots: Cannot be stolen.

Selected weapons and ammunition: Cannot be stolen.

Not selected weapon: 95.

Gloves, amulet, belt and cloak: 80.

Ring: 60.

Not selected ammunition and quick items: 50.

Other inventory slots and gold: 10.

How did I do? Does my translation have any relation with the original at all?

The rule of thumb I use for Thieving skills is that there is no point to increasing your Open Lock, Find Traps, Set Snare, and Detect Illusion skills beyond 100, as 100 basically guarantees success. Hide in Shadows, Move Silently and Pickpocketing are the only Thief skills that have a great deal of potential modifiers, so feel free to continue plugging your excess points into those 3 (although I've yet to encounter anyone that I couldn't pickpocket with 100 + a Potion of Master Thievery).

do you mean the potions? there are, at least in bg2, 2 kind of potions that improve the thieving skills, and the effect is to add temporarily points to certain thieving skills like is clearly stated in the potion description.
Potion of Master Thievery
Properties:
+40% Bonus to Lock Picking.
+40% Bonus to Pick Pockets.
Duration: 3 hours.
Usable by: Bards and Thieves.

the effect stack with the other potion, with the same potion and with items.
so a bard with 80 points in pick pocketing that drinks 2 potions of master thievery has 160 points in pick pocket (80base + 40 + 40) until the potions effect expires or is dispelled.
a thief with 50 in lock pick that drink 1 of each kind of potion get 110 (50+40+20).
so the modifier is not applied to the formula that check if the action is successful, but directly to the value of the thieving skill that is used in the formula.

there are other potions that rise dex, and afaik also it can be beneficial for those thieving skills that benefit of a good dex, if a dex value enough high to rise their score is reached.

if ee has not changed it boosting a thieving skill further then the maximum value allowed by the system, that should be 255 or 256, i am not sure which one, is detrimental as the value "turns around the clock" and you get only the points that exceed the max value. so if a thief has a natural 220 in open locks and drink a potion of master thievery he does not get 260 points in the skill, but 260-256 (255)=4( 5).
this is important only if a thief try to boost a skill near to the maximum possible value, is a very rare occurrence, but can happen with a thief with a really good score over buffing with items and potions.

I guess this means "the result of a 1d10 roll adjusted by luck". If that's correct, how does luck adjust the roll? I've read @Bubb's description in the luck thread, but I don't understand it.

It's the same principle as for calculating a damage roll - each point of luck adds to the actual die roll made, but the result can never go about the maximum. If you have one point of luck a die roll of 1 will become a 2, a 2 becomes 3 etc up to 9 and 10 which both become 10.

The adjusted score is then added to your skill level and compared with the requirement to see if you're successful. For instance if you have a score of 50 in detect/disarm traps and are trying to disarm a trap with a difficulty level of 60, then the random roll (adjusted by luck) is checked to see if it equals or exceeds 60. With no luck that means you would succeed 10% of the time. With 1 point of luck you would succeed 20% of the time. If you somehow had 9 points of luck you would succeed every time.

@gorgonzola negative luck will affect the d10 modifier, yes. Rather than each point of positive luck adding 1 to the die roll with a maximum of 10, each point of negative luck will reduce the die roll with a minimum of 1. In the above example with a skill of 50 and a difficulty of 60, you would never be successful with a negative point of luck as a roll of 10 would be reduced to 9.

I have written a description of everything we've discussed in this thread so far. I hope I got it right. Let me know what you think:

When a rogue attempts to use one of his thieving abilities, his chances of success depend on several factors: his skill, the difficulty of the task at hand, and sometimes also his luck. Each thieving task has an associated difficulty score. For example, very complex locks or traps have higher difficulty scores than simpler ones.

The method used to determine the result (success or failure) of a thieving task is different for each thieving ability.

FINDING TRAPS, DISARMING TRAPS AND OPENING LOCKS
When the detect traps/illusions mode of a thief is active and there are traps within his line of sight, the game follows these steps to determine whether he detects each trap:

Roll a d10 and adjust the result with the rogue’s luck. This means that the luck score is added to the die roll, but results higher than 10 are reduced to 10, and results lower than 1 are increased to 1. Examples:

The luck of the rogue is 2. His die roll is 5. The adjusted result is 5 + 2 = 7.

The luck of the rogue is 2. His die roll is 9. 9 + 2 = 11, which is higher than 10, so the result is reduced to 10.

The luck of the rogue is -2. His die roll is 1. 1 - 2 = -1, which is lower than 1, so the result is increased to 1.

Add the adjusted result of the roll to the Detect traps score of the rogue.

Compare (2) with the difficulty of each trap. If (2) equals or exceeds the difficulty, the rogue detects the trap. If it’s lower, the rogue misses it.

The game keeps performing this check once per round while the detect traps/illusions mode is active and there are traps nearby.

Example: A rogue with a score of 50 in Detect traps and +2 in luck is close to two traps with difficulty scores of 55 and 60. He activates his detect traps mode. The game rolls 1d10 and the result is 4. The roll adjusted with luck is 4 + 2 = 6. The adjusted roll is added to the detect traps score, for a total of 56. 56 is higher than the difficulty of the first trap (55), but lower than the difficulty of the second one, so the rogue detects only the first trap. The rogue keeps detecting. The game rolls again, this time the result is 9. The roll adjusted with luck is 10 (9 + 2 = 11, which is reduced to 10). The adjusted roll is added to the detect traps score, for a total of 60. 60 equals the difficulty of the second trap, so the rogue detects it as well.

The same method is used to determine the result when a rogue attempts to disarm a trap or open a lock.

DETECTING ILLUSIONS AND SETTING TRAPS
The Detect illusions and Set traps abilities work in a similar way to the previous ones, but there are two important differences: The difficulty of the task is always 100, and the die rolled is a d100, which means that there is much more randomness involved.

When a rogue attempts to detect illusions the game rolls 1d100 and adds the result to his Detect Traps/Illusions score. If the result is lower than 100, the detection fails. Otherwise, the detection succeeds, all the Illusion spells
within visual range of the character are dispelled, and any illusory creatures in that range are destroyed.

Set traps works in the same way, but the die roll is adjusted with luck (see Detecting traps, disarming traps and opening locks above).

STEALTH
Just like with detecting illusions and setting traps, the difficulty of stealth is always 100. The calculation, however, is more complex. The game makes this calculation:

1d100 MOD luck is the result of rolling 1d100 and adjusting the roll with luck (see Detecting traps, disarming traps and opening locks above). If the result of this adjusted roll is 1, it is a critical failure. In this case, the stealth attempt fails regardless of the skill scores.Environment Multiplier is a number that depends on how well lit the area is. Darker areas have higher values.

If the result of the calculation equals or exceeds 100, the stealth attempt succeeds, otherwise it fails.

Critical failures: If the result of the adjusted roll is 1, it is a critical failure. In this case, the stealth attempt fails regardless of the skill scores of the rogue. Rogues with positive luck (1 luck or higher) never get critical failures, since they cannot possibly get 1 in the adjusted roll. Rogues with neutral (0) luck have a 1% chance of critical failure. Each point of negative luck increases the chance of critical failure by 1%. For example, a rogue with -2 luck has a 3% chance of critical failure.

PICKING POCKETS
When a rogue attempts to steal an item from a creature or a store, three results are possible:

Failure: The target notices the stealing attempt and turns hostile, and the rogue doesn't manage to steal anything.

The outcome is determined in differents ways depending on whether the rogue is stealing from a store or a creature.

Stealing from a store
The game makes this calculation:

(Thief skill - target skill) / 5 vs. 1d20

I. e., it calculates the difference between the Pick pockets score of the thief and the Pick pockets score of his target, divides it by 5, and compares the result with a d20 roll. If the result is equal or greater than the d20 roll, the attempt succeeds. Otherwise it fails (and the target turns hostile).

Stealing from a creature
The process is more complex when stealing from creatures. The game starts by doing the same calculation described for stores, but in this case a roll of 20 is a critical failure, i. e., the attempt fails regardless of the skill of the rogue. Just like with store stealing, if the d20 roll check fails, the attempt fails and the target turns hostile. However, if the d20 roll check succeeds, it doesn't necessarily mean that the rogue manages to steal something, it just means that his attempt goes unnoticed. The game still needs another step to determine whether he actually steals something.

Each of the inventory slots of a creature has an associated pickpocketing difficulty score. If the target has an item in an inventory slot with a difficulty score equal or lower than the Pick pockets score of the rogue, he steals that item. Otherwise you get a message informing you that the target has no items a thief of your ability can get (and nothing happens).

Example: A thief with a Pick pockets score of 50 attempts to steal from a noble (Pick pockets score 0) who is wearing a ring on his finger. The game rolls a d20 and the result is 7. The first calculation ((Thief skill - target skill) / 5) yields: (50-0)/5=10. This is greater than the d20 roll, so the attempt doesn't fail. However, the pickpocketing difficulty score of worn rings (i. e., rings located in one of the ring slots of the inventory), is 60. The pickpocketing skill of the thief is lower than this, so he doesn't steal the ring. Then the thief drinks a potion of Master thievery, which raises his score to 90, and tries again. The game rolls a d20 and the result is 7 again, so he still avoids failure. This time his score is higher than the difficulty score of worn rings, so he succeeds and steals the ring.

The pickpocketing difficulty score of each inventory slot is:

Helmet, armor, shield and boots: Cannot be stolen.

Selected weapons and ammunition: Cannot be stolen.

Not selected weapon: 95.

Gloves, amulet, belt and cloak: 80.

Rings: 60.

Not selected ammunition and quick items: 50.

Other inventory slots and gold: 10.

Hostile creatures cannot be pickpocketed. However, using a charm like spell to make them friendly makes them vulnerable to pickpocketing.

Some conclusions

A rogue needs a Pick pockets score at least 5 points higher than his target just to have any chance of success at all. If the difference is lower, his stealing attempts will automatically fail.

A rogue attempting to steal from a store has guaranteed success if his Pick pockets score is at least 100 points higher than the Pick pockets score of the seller.

There's no way to have guaranteed success when stealing from a creature.

As I wrote this, a lot of questions popped in my mind:

At what distance can a thief detect traps?

How long does it take to detect a trap or an illusion from the moment the detect traps/illusions mode is activated? If the first attempt fails, how long does it take until there is a second attempt?

Do traps have different difficulty scores for detection and disarming, or is it the same for both?

What abilities are affected by critical success (I guess that’s the right word to use here?) or critical failure? Bubb explains here how luck affects critical failures for stealth, but he doesn’t say anything about critical success or the other abilities.

What happens when the difficulty of a trap or lock is 100? Is it impossible to detect/disarm/pick it?

How does pickpocketing work in stores? Do they have a set difficulty score for all their items?

Detect illusions: Is there a single check for all the illusions in the area, or does each illusion receive its individual check?

Detect illusions: What happens when there are “nested” illusions? For instance, a mage casts Mirror image and then
Invisibility. Are both dispelled at the same time or do they require successive detections?

What are the values of the environment multipliers for the stealth ability?

Regarding 7 and 8: I believe that each enemy with Illusion spells active gets a different check for a player's Detect Illusions skill. I know that if Detect Illusion works, it will remove all Illusion spells, whether it's Mirror Image or Shadow Door (and I think even non-beneficial spells like Blindness, since True Seeing can cure Blindness spells on the other team).

That check is still there to determine success or failure of the attempt (where failure normally turns the target hostile). However, Beamdog have introduced a further check in the EE in the form you outlined. It's quite possible for your pickpocket attempt to succeed, but for you to be told that the target has no items a thief of your ability can get. You might also want to mention that hostile creatures can't be pickpocketed (but you can charm them first if you want to).

For stealth the calculation takes the average of HIS and MS. As you noted in your questions a roll of 100 is a critical failure for stealth, even if your score is above that. However, luck modifications apply here and, unlike the situation with THAC0, a luck modifier will reduce the roll before considering critical failure. Hence a single point of positive luck means you never suffer stealth failure if your score (adjusted by the environment modifier) is 100 or more.

For 2. the check takes place once a round.

For 3. the scores can be different.

For 4. and 6. the calculation is the same in stores as for pickpocketing individuals. However, in stores there is no critical failure on a roll of 20 - thus if your skill is 100 or more greater than the storekeeper's you will never fail when stealing from stores.

Beamdog have introduced a further check in the EE in the form you outlined. It's quite possible for your pickpocket attempt to succeed, but for you to be told that the target has no items a thief of your ability can get.